Abstract

A sensor based on rotational signatures in the submillimeter (SMM) region is described. This sensor uses frequency synthesis techniques in the region around 10GHz, with nonlinear diode frequency multiplication to 210–270GHz. This provides not only a nearly ideal instrument function, but also frequency control and agility that significantly enhance the performance of the spectrometer as a sensor. The SMM frequencies provide significantly stronger absorptions and broader spectroscopic coverage than lower-frequency microwave systems. Among the characteristics of the sensor are absolute specificity, low atmospheric clutter, good sensitivity, and near-term paths to systems that are both compact and very inexpensive.

Figures (3)

50GHz of a spectrum of a mixture of 20 gases on a highly compressed scale taken with a FASSST backward-wave-oscillator-based system (top). 0.4% of this spectrum is expanded to show individual lines (bottom). Comparison with the library spectra of eight gases shows their presence in the gas mixture (middle).

Overview of a SMM frequency multiplication, heterodyne receiver gas sensor. In this system a frequency synthesizer near 10GHz drives a 24× diode-based frequency multiplier to provide the local oscillator of a heterodyne receiver. A sideband generation scheme provides synchronized, but offset in frequency, drive to a similar multiplier chain for probe power to the folded 1.2m absorption cell.

Observed spectra at each of the six fingerprint regions selected for each gas for the family of 32 gases considered, with enlargements of specific fingerprint regions. Right, results of a quantitative LSQ analysis. A digital lock-in recovers a near first derivative line shape that results from a small FM modulation on the probe driver.